1. Field of the Invention
The present invention relates to an intake and/or exhaust-valve timing control system which is optimally adapted for use in internal combustion engines, and specifically to a system which is variably capable of controlling intake- and/or exhaust-valve timing depending upon the operating state of the engine, for example, the magnitude of engine load and/or engine speed.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various intake and/or exhaust-valve timing control systems for internal combustion engines for generating optimal engine performance according to the operating state of a vehicular engine.
As is generally known, valve timing is usually determined such that optimal engine performance s obtained; however, a predetermined valve timing is not suitable under all operating conditions. For example, when an engine is operating within a range of low engine revolutions, higher torque will be obtained with an intake-valve timing earlier than a fixed, predetermined valve timing.
Such conventional intake- and/or exhaust-valve timing control systems for internal combustion engine have been disclosed in U.S. Pat. Nos. 4,231,330 and 4,535,731. In these conventional valve timing control systems, a cam sprocket is rotatably supported through a ring gear mechanism by the front end of a camshaft. The ring gear mechanism includes a ring gear having an inner toothed portion engaging another toothed portion formed on the front end of the camshaft and an outer toothed portion engaging an inner toothed portion formed on the inner peripheral wall of the cam sprocket. In this manner, the ring gear rotatably engages between the cam sprocket and the camshaft. The ring gear is normally biased in the axial direction of the camshaft by means of a return spring, such as a coil spring. At least one of the two meshing pairs of gears is helical. The result is that axial sliding movement of the ring gear relative to the camshaft causes the camshaft to rotate about the cam sprocket and therefore the phase angle between the camshaft and the cam sprocket (and consequently, the phase angle between the camshaft and the engine crankshaft) is relatively varied. The ring gear moves, as soon as one of the two opposing forces acting on it, namely the preloading pressure of the above spring means or the oil pressure applied from the oil pump to the ring gear, exceeds the other. In the previously noted conventional valve timing control systems, a hydraulic circuit which serves as a ring gear driving hydraulic circuit, functions to feed a controllable oil pressure to a pressure chamber defined at the one end of the ring gear, and to feed an engine lubricating oil for lubricating rotational friction surfaces between a cylinder head, a bearing member, and a camshaft journaled by the cylinder head and the bearing member, and in addition to feed working fluid for operating a hydraulically operated valve lifter. In other words, a single hydraulic circuit is commonly utilized to provide the axial sliding movement of the ring gear, and to lubricate the friction surfaces between the camshaft, the cylinder head and the bearing member, and in addition to operate the valve lifter.
In this construction of the conventional variable valve timing control system, since a portion of working fluid discharged from the oil pump must be used for lubricating the rotational friction surfaces, even during low engine revolutions, wherein the working fluid pressure discharged from the oil pump is relatively low, a sufficient amount of working fluid is not supplied for the ring gear driving hydraulic circuit for the variable valve timing control system, during low engine speed and high engine load. Under this condition, since the hydraulic pressure supplied in the fluid cannot sufficiently overcome the biasing force caused by the return spring with the result that the ring gear is not quickly moved against the spring force but moderately moved according to a relatively low working fluid pressure. During low engine speed and high engine load, quick, axial sliding movement of the ring gear is prevented due to a low working fluid pressure supplied to the pressure chamber. Consequently, a prior art variable valve timing control system tends to exhibit a low step-response with regard to an intake- and/or exhaust-valve timing control executed by a variable valve timing control system, during low engine speed and high engine load.